Re: PG 12 draft release notes

On Mon, May 20, 2019 at 3:17 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> <!--
> Author: Alexander Korotkov <akorotkov(at)postgresql(dot)org>
> 2018-07-28 [d2086b08b] Reduce path length for locking leaf B-tree pages during
> Author: Peter Geoghegan <pg(at)bowt(dot)ie>
> 2019-03-25 [f21668f32] Add "split after new tuple" nbtree optimization.
> -->
>
> <para>
> Improve speed of btree index insertions (Peter Geoghegan,
> Alexander Korotkov)
> </para>

My concern here (which I believe Alexander shares) is that it doesn't
make sense to group these two items together. They're two totally
unrelated pieces of work. Alexander's work does more or less help with
lock contention with writes, whereas the feature that that was merged
with is about preventing index bloat, which is mostly helpful for
reads (it helps writes to the extent that writes are also reads).

The release notes go on to say that this item "gives better
performance for UPDATEs and DELETEs on indexes with many duplicates",
which is wrong. That is something that should have been listed below,
under the "duplicate index entries in heap-storage order" item.

> Author: Peter Geoghegan <pg(at)bowt(dot)ie>
> 2019-03-20 [dd299df81] Make heap TID a tiebreaker nbtree index column.
> Author: Peter Geoghegan <pg(at)bowt(dot)ie>
> 2019-03-20 [fab250243] Consider secondary factors during nbtree splits.
> -->
>
> <para>
> Have new btree indexes sort duplicate index entries in heap-storage
> order (Peter Geoghegan, Heikki Linnakangas)
> </para>

> I'm not sure that the grouping here is quite right. And the second entry
> probably should have some explanation about the benefits?

It could stand to say something about the benefits. As I said, there
is already a little bit about the benefits, but that ended up being
tied to the "Improve speed of btree index insertions" item. Moving
that snippet to the correct item would be a good start.

--
Peter Geoghegan